A synchronous digital communication system operates, for example, according to a standard for synchronous digital hierarchy (SDH/SONET standard) as set forth, for example, in ANSI T1.105-1991 entitled "Digital Hierarchy-Optical Interface Rates and Formats Specifications (SONET)". In such a digital transmission system, individual network elements are interconnected by different transmission media (e.g., copper cables, optical fiber waveguides, or radio links).
The connection between a network element and a transmission medium is provided by interface units ("network node interfaces"), which are known, for example, from ITU-T Recommendations G.703 and G.957. Recommendation G.703 specifies electrical characteristics of such interface units, and Recommendation G.957 specifies optical characteristics of such units. In an interface unit connected to an optical fiber waveguide, incoming optical signals, for example, are converted to electric signals by an optical-to-electrical transducer.
A network element is, for example, an exchange for a public switched telephone network, or a cross-connect or an add/drop multiplexer. A switching principle used in the exchange requires that all exchanges in the system operate synchronously. A general account of how exchange can be synchronized is given in an article by W. E. Powell et.al, "Synchronization and Timing of SDH Networks", Electrical Communication (Alcatel) , 4th Quarter, 1993, pages 349 to 358.
In that article, two techniques for synchronizing the network elements are mentioned: master-slave synchronization and mutual synchronization.
The master-slave technique applies a unique primary reference clock for synchronization of a first hierarchical level of nodes. These nodes give their derived clocks to the next level nodes, and so on. In the mutual synchronization technique, all nodes are at a peer level interconnected by the existing digital links. Each node calculates a mean phase value of the incoming clocks and its own internal clock.
The master-slave technique thus uses a hierarchical synchronization system. In a system according to the synchronous digital hierarchy (SDH) or Sonet standard, a frame is defined which has an area referred to as "section overhead" (SOH) for SDH, in a multiplex section. In this SOH, a synchronization quality marker is transmitted, for which bits 5 to 8 of the S1 byte are allocated (see, for example, ITU-T Recommendation G.707, G.708, or Similarly, according to Committee T1-Telecommunications sponsored by the Alliance for Telecommunications Industry Solutions (formerly the Exchange Carriers Standards Association), G.709).
This synchronization quality marker indicates a quality class of a transmitted standard reference clock and provides a powerful autonomous capability to enhance the management of synchronization quality. The synchronization quality marker is generally called "synchronization status message" and will hereinafter be referred to as SSM. Bits 5 to 8 of the S1 (Z1) byte define, among other things, standard reference clocks according to ITU-T Recommendations G.811 and G.812 or a "do not use for synchronization" message, henceforth called DNU.
From a working document by B. Neihoff, "More on Synchronization Status Message Problems", ETSI TM3/WG6, Working Document 22, Oslo, 25-29 Oct. 1993, it is known that problems may be encountered in the use of SSMs. These include timing loops, which are created if, as shown in FIG. 2 of that document, a network element (SDXC) loses its selected reference clock source and selects another reference clock source with the aid of a selection procedure (synchronization source selection algorithm, SSSA). It is also mentioned that such timing loops can be created in any network configuration where two bidirectional SSM paths terminate and are generated in the same network element. To solve the problem, the working document proposes to define enable/disable functions.